1. Field of the Invention
This invention relates to a method for measuring oxidation-reduction potential in a flue gas desulfurization process which method is adaptable for use with a method for controlling the oxidation of sulfites in a flue gas desulfurization process wherein the oxidation of calcium sulfite in an absorbing fluid can be achieved efficiently.
2. Description of the Related Art
When exhaust gas containing sulfur oxides is subjected to flue gas desulfurization according to the wet lime-gypsum method, sulfur dioxide which is a predominant sulfur oxide present in the exhaust gas is brought into contact with an absorbing fluid containing calcium carbonate and absorbed according to the following reaction. EQU SO.sub.2 +CaCO.sub.3 .fwdarw.CaSO.sub.3 +CO.sub.2
A portion of the calcium sulfite so produced is oxidized by oxygen present in the exhaust gas to form gypsum, as represented by the following reaction formula. EQU CaSO.sub.3 +1/2O.sub.2 .fwdarw.CaSO.sub.4
Usually, the oxygen concentration in the exhaust gas is so low that the oxidation of calcium sulfite to gypsum is not sufficiently effected. Accordingly, an oxygen-containing gas is supplied from the outside of the system and passed through the absorbing fluid.
However, if the flow rate of the oxygen-containing gas is low, the concentration of unoxidized calcium sulfite will increase. This may cause several difficulties including an inhibition of the dissolution of calcium carbonate used as the absorbent, a reduction in desulfurization performance, and an increase in the chemical oxygen demand (hereinafter referred to as "COD") of waste water from the desulfurizer.
On the other hand, if an attempt is made to maintain a high degree of conversion of calcium sulfite to gypsum, it is inevitable to supply the oxygen-containing gas in excess with consideration for load fluctuations and the like. This leads to an increase in running cost and a rise in the COD of waste water.
Accordingly, it is necessary to control the flow rate of the oxygen-containing gas so as to remain in a proper range.
In order to control the flow rate of the oxygen-containing gas involved in the oxidation of calcium sulfite, a method based on the use of oxidation-reduction potential (hereinafter referred to as "ORP") is known. In the conventional method for controlling the flow rate in response to ORP, a preset ORP value is determined in advance on the basis of the preestablished relationship between ORP and sulfurous acid concentration, and the flow rate is controlled in response to a deviation signal between a signal obtained by detecting the ORP of the absorbing fluid continuously and the preset ORP value.
However, ORP is affected not only by sulfurous acid concentration, but also by pH and dissolved solution components. Consequently, the conventional method has the disadvantage that stable oxidation control cannot be achieved because of variation in pH and changes of dissolved solution components, which result from load fluctuations, changes of the absorbent material, and/or changes of the type of fuel, as well as erroneous indications of the pH meter. This may cause such difficulties as an increase in the COD of waste water due to an increase in sulfurous acid concentration or an oversupply of air.
In order to overcome these disadvantages, the present inventors have developed an oxidation controlling method which comprises continuously detecting a first deviation signal between the ORP of the absorbing fluid and the ORP of the absorbing fluid in a completely oxidized state by means of an ORP detector equipped with a sample fluid tank for detecting the ORP of the absorbing fluid and a reference fluid tank for oxidizing the absorbing fluid by the passage of air therethrough and detecting the ORP of the absorbing fluid in a completely oxidized state, and controlling the flow rate of the oxygen-containing gas in response to a second deviation signal between the first deviation signal and a preset ORP deviation value (Japanese Patent Provisional Publication No. 24566/1996).
One example of an ORP detector constructed on the basis of this method is illustrated in FIG. 3, and the method for measuring ORP is described below with reference to this figure. From an absorption tower where combustion exhaust gas is brought into contact with an absorbing fluid containing a calcium compound, a portion of absorbing fluid 3 is introduced into an ORP measuring tank 17. ORP measuring tank 17 is partitioned into a sample fluid tank 18 and a reference fluid tank 19. In reference fluid tank 19, the absorbing fluid is completely oxidized by supplying air 20 from the outside of the system. In these tanks 18 and 19, the ORP of the absorbing fluid and the ORP of the absorbing fluid in a completely oxidized state are detected by ORP electrodes 21 and 22, respectively. The detected signals are fed to an arithmetic unit 23 where the deviation between the ORP of the absorbing fluid and the ORP of the absorbing fluid in a completely oxidized state is calculated. A deviation signal 24 representing this deviation is delivered from arithmetic unit 23. After ORP measurements are made, the return absorbing fluid from sample fluid tank 25 and the return absorbing fluid from reference fluid tank 26 are returned again to the fluid reservoir of the absorption tower.
In this method, the ORP of the absorbing fluid is continuously detected in one (i.e., sample fluid tank 18) of the two tanks into which the ORP measuring tank is partitioned. In the other tank (i.e., reference fluid tank 19), the ORP of the absorbing fluid in a completely oxidized state is continuously measured by constantly passing air through the absorbing fluid placed therein. However, since peroxides may be present in the absorbing fluid according to the operating condition of the desulfurizer, the values obtained by passing air through the absorbing fluid and measuring the ORP of the absorbing fluid in a completely oxidized state become unstable. This may interfere with the maintenance of stable oxidation control and cause such difficulties as an increase in the COD of waste water due to an oversupply of air.